One question that has endlessly fascinated ecologists and evolutionary biologists is why there is so much biotic diversity on earth. At the advent of the field, Darwin grappled with these questions to propose that trade-offs between fitness traits constrained the evolution of hypothetical all-fit organisms termed ‘Darwinian demons’. Trade-offs in the co-evolution of antagonistic biotic partners are thought to be particularly important for diversification processes. Despite the centrality of trade-off theory to evolutionary biology, the genetic underpinnings of and selection dynamics on trade-offs remain poorly understood.
My dissertation therefore focuses on trade-offs: what forms them, what influences them, and how they interact. To do this, I use experimental evolution methods in the Plodia interpunctella, or Indian meal moth, and granulosis virus model system. I focus on two trade-offs—one for each partner in a host-parasite interaction.
On the host side, I focus on the trade-off to resistance to viral in infection. Understanding this trade-off is important for predicting when we expect to see resistance evolve and how it would alter ecological dynamics or persist in the absence of the pathogen. In chapter 1 of the dissertation, I ask whether the trade-off to resistance is symmetric such that selection for the longer development time phenotype constituting a cost to resistance produces symmetric gains in resistance. In chapter 2 of the dissertation, I ask why resistance inconsistently evolves in the system and how time scales and resource levels affect such resistance evolution and its costs. With these two chapters, I find that the shape of the trade-off between resistance to infection and development time can change depending on the population’s specific selection conditions when trade-offs are influenced by many genes. This is important because such differences in trade-off shape would alter the outcome of evolution.
On the virus side, I examine how trade-offs between adaptation to different host genotypes influences host range evolution in our system. This trade-off is important because specificity in biotic interactions promotes diversity in co-evolutionary systems. In chapter 3, I examine the genetic and phenotypic dynamics of host genotype specialization in the granulosis virus. In chapter 4, I experimentally evolve granulosis virus in mesocosms of its Plodia interpunctella host with varying degrees of spatial structure and host genetic diversity. With these two chapters, I show that trade-offs between host genotypes might not follow simple functions and may depend on their interactions with other selection pressures. This is important because it suggests that costless generalism can exist in some evolutionary and ecological scenarios, thus interrupting evolutionary dynamics that depend on specialist interactions.
